EP0995986A2 - Apparat zum Messen von Gaskonzentrationen - Google Patents

Apparat zum Messen von Gaskonzentrationen Download PDF

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Publication number
EP0995986A2
EP0995986A2 EP99118691A EP99118691A EP0995986A2 EP 0995986 A2 EP0995986 A2 EP 0995986A2 EP 99118691 A EP99118691 A EP 99118691A EP 99118691 A EP99118691 A EP 99118691A EP 0995986 A2 EP0995986 A2 EP 0995986A2
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EP
European Patent Office
Prior art keywords
cell
sensor
gas concentration
voltage
gas
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP99118691A
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English (en)
French (fr)
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EP0995986A3 (de
Inventor
Tomoo c/o Denso Corporation Kawase
Eiichi c/o Denso Corporation Kurokawa
Satoshi c/o Denso Corporation Hada
Toshiyuki c/o Denso Corporation Suzuki
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Denso Corp
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Denso Corp
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Priority to EP06125003.1A priority Critical patent/EP1764613B1/de
Publication of EP0995986A2 publication Critical patent/EP0995986A2/de
Publication of EP0995986A3 publication Critical patent/EP0995986A3/de
Ceased legal-status Critical Current

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/4067—Means for heating or controlling the temperature of the solid electrolyte
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
    • G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
    • G01N33/0037—NOx
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

  • the present invention relates to a gas concentration sensing apparatus using a gas concentration sensor capable of detecting the concentration of a specific component contained in an exhaust gas emitted from an automotive engine.
  • a combustion control monitoring or a catalyst monitoring becomes feasible by directly monitoring a NOx concentration in the exhaust gas, this will make it possible to accurately check the reduction of the pollution substances in the exhaust gas. Namely, if the fuel injection or the EGR rate is feedback controllable by a NOx concentration value detected from the exhaust gas, it will become possible to reduce the pollution substances emitted from engines. Furthermore, providing a NOx sensor at a downstream side of the catalytic converter is effective to easily check a deteriorated condition of a catalyst accommodated in the catalytic converter.
  • This kind of conventional technique is, for example, disclosed in the unexamined Japanese patent publication No. 8-271476 (corresponding to the US Patent No. 5,866,799) or in the unexamined Japanese patent publication No. 9-318596 (corresponding to the European Patent Application No. 798,555), according to which the oxygen concentration is reduced in advance to detect the NOx concentration. More specifically, a chamber is provided separately from the exhaust gas environment via a diffusive resistor. A pump cell discharges the oxygen (O2) from this chamber. A sensor cell decomposes and discharges the residual NOx. A current value obtainable from the sensor cell indirectly represents the NOx decomposition amount, i.e., the NOx concentration.
  • the NOx sensor comprises a heater provided in the vicinity of the pump cell and the sensor cell for warming up these cells.
  • the NOx sensor when detecting the exhaust gas components, the NOx sensor is subjected to a large temperature fluctuation as well as a large gas flow fluctuation of the exhaust gas. Accordingly, to improve the accuracy of the NOx sensor, it is necessary to accurately control the temperature of the NOx sensor to a constant value.
  • a first method for controlling a sensor temperature is to detect a heater resistance and control the sensor temperature in accordance with a detected heater resistance value.
  • a heater temperature is measured indirectly by measuring the heater resistance value. The measured temperature is regarded as being substantially equal to the cell temperature. And, the temperature control is performed so as to maintain the heater temperature at a constant level.
  • a second method is to detect an internal impedance of a pump cell and control the sensor temperature in accordance with a detected internal impedance value.
  • an impedance of the pump cell is detected to control the heater.
  • SAE-No. 970858 discloses a heater control of a sensor comprising first and second pump cells. According to this conventional heater control, an impedance of the second pump cell is detected and the heater is controlled based on a detected impedance value.
  • the above-described first method is inaccurate in controlling the sensor cell temperature when the exhaust gas temperature varies widely or the exhaust gas flow speed is high, because the cell temperature is different from the heater temperature in such conditions.
  • the above-described second method is inaccurate in controlling the sensor cell temperature when the exhaust gas temperature varies widely or the exhaust gas flow speed is high, because the pump cell temperature is different from the sensor cell temperature in such conditions.
  • the sensor cell temperature fluctuates and the NOx concentration sensing accuracy worsens. It is thus desirable to provide a sensing apparatus capable of accurately detecting a gas concentration regardless of the temperature fluctuation or gas flow speed change in the exhaust gas.
  • the present invention has an object to provide a gas concentration sensing apparatus which always assures an accurate gas concentration detection regardless of the temperature fluctuation or gas flow speed change in the measuring gas.
  • the present invention provides a gas concentration sensing apparatus using a gas concentration sensor comprising a first cell for discharging excessive oxygen contained in a measuring gas in accordance with an applied voltage and producing a current responsive to an oxygen concentration, a second cell producing a current responsive to a concentration of a specific component involved in the residual measuring gas after the excessive oxygen is discharged, and a heater for heating the first cell and the second cell.
  • a gas concentration sensing apparatus when this gas concentration sensing apparatus is used to detect both the oxygen concentration and the NOx concentration in an exhaust gas, the first cell detects the oxygen concentration and the second cell detects the NOx concentration.
  • an internal resistance of the second cell is detected. And, electric power supplied to the heater is controlled in accordance with a detected internal resistance value of the second cell.
  • the above-described arrangement makes it possible to control the internal resistance of the second cell to a desired value constantly through the electric power control of the heater. Accordingly, it becomes possible to prevent the temperature of the second cell from being changed undesirably due to the temperature or flow speed fluctuation of the measuring gas (e.g., exhaust gas), thereby appropriately maintaining the NOx concentration sensing accuracy. Furthermore, as apparent from the characteristics shown in Fig. 6, the NOx concentration is detectable in a relatively narrow region (i.e., a flat region). The temperature variation of the second cell (e.g., a sensor cell) renders the sensor output unstable.
  • the above-described heater power control ensures an accurate detection of the NOx concentration. As a result, the present invention makes it possible to always assure an accurate gas concentration detection regardless of the temperature fluctuation or the gas flow speed change of the measuring gas.
  • an internal resistance of the first cell is also detected.
  • the voltage applied to the first cell is controlled in accordance with a detected internal resistance value of the first cell.
  • the temperature of the second cell e.g., sensor cell
  • the temperature of the first cell e.g., pump cell
  • the oxygen concentration sensing accuracy can be maintained appropriately.
  • the sensing accuracy of the NOx concentration is also improved in accordance with the improvement of the oxygen concentration sensing accuracy.
  • the present invention further provides a second gas concentration sensing apparatus using a gas concentration sensor comprising a plurality of cells including a first cell for discharging excessive oxygen contained in a measuring gas in accordance with an applied voltage and producing a current responsive to an oxygen concentration, and a second cell producing a current responsive to a concentration of a specific component involved in the residual measuring gas after the excessive oxygen is discharged, and a heater for heating the plurality of cells.
  • the second gas concentration sensing apparatus is characterized by detecting means for detecting an internal resistance of each of the plurality of cells, judging means for judging temperature conditions of the plurality of cells, and power control means for selectively performing a heater power control based on a detected internal resistance value with reference to the judgement result of the temperature conditions.
  • the heater power control is performed based on a detected internal resistance value of a highest temperature cell among the plurality of cells. And thereafter, the heater power control is performed based on a detected internal resistance value of the second cell. Alternatively, it is desirable that the heater power control is performed based on a detected internal resistance value of a highest temperature cell among the plurality of cells when there is a large temperature difference among the plurality of cells. And, the heater power control is performed based on a detected internal resistance value of the second cell when there is a small temperature difference among the plurality of cells.
  • the heater power control is performed based on the internal resistance of the second cell.
  • the heater power control is performed based on the internal resistance of the cell having the highest cell.
  • a third gas concentration sensing apparatus in accordance with the present invention comprises first detecting means for detecting an internal resistance of the first cell, second detecting means for detecting an internal resistance of the second cell, and power control means for controlling electric power supplied to the heater so as to equalize a sum or an average of detected internal resistance values of the first and second cells with a target value.
  • the second or third gas concentration sensing apparatus may further comprise voltage control means for controlling the voltage applied to the first cell based on the detected internal resistance value of the first cell. In this case, it is preferable to control the voltage applied to the first cell in accordance with the detected internal resistance value of the first cell.
  • the applied voltage can be adequately managed.
  • the oxygen concentration sensing accuracy can be maintained appropriately.
  • the internal resistance of each cell is detected by temporarily changing the voltage or current applied to each cell.
  • a sample hold circuit is provided in a signal path for outputting a sensor signal representing a detected oxygen or other gas concentration in the measuring gas.
  • the sample hold circuit holds a latest value of the sensor signal during the internal resistance detection of each cell.
  • the gas concentration sensing apparatus temporarily changes the voltage applied to respective cells including the first and second cells to detect their internal resistance values. Such a voltage change may cause an interference of currents flowing through respective cells.
  • the output signal representing a detected gas concentration may fluctuate undesirably.
  • the present invention provides the sample hold circuit for holding the latest value of the sensor signal during the internal resistance detection of each cell.
  • the first to third gas concentration sensing apparatus may further comprise speed limiting means for limiting a change speed of the voltage applied to each cell. This is effective to suppress the oscillation of the applied voltage.
  • a gas concentration sensing apparatus of the first embodiment is applicable to a gasoline engine for an automotive vehicle.
  • An air-fuel ratio control system incorporated in this engine, performs a feedback control for obtaining a desirable air-fuel ratio (A/F) by controlling a fuel injection amount supplied to the engine based on a detected value of the gas concentration sensing apparatus.
  • the first embodiment obtains gas concentration data from a so-called combined or composite gas sensor.
  • the combined gas sensor is capable of simultaneously detecting an oxygen concentration and a NOx concentration from an exhaust gas.
  • the gas concentration sensing apparatus of the first embodiment performs an air-fuel ratio feedback control based on a detected oxygen concentration, and also controls a NOx catalyst (e.g., NOx adsorbable and reducible catalyst) installed in an engine exhaust pipe in accordance with a detected NOx concentration.
  • NOx catalyst control is performed in the following manner. Some of NOx gas is discharged without being purified by the NOx catalyst. This amount can be indirectly judged from the detected value of a gas concentration sensor. When an increase of a non-purified NOx amount is detected, the reconstructive processing is performed to restore the NOx purification ability. For example, a rich gas is temporarily supplied to the NOx catalyst to remove the adsorbed ions from the catalyst.
  • FIG. 1 shows a schematic arrangement of a gas concentration sensing apparatus in accordance with the first embodiment.
  • a gas concentration sensor 100 comprises a pump cell 110 detecting an oxygen concentration, a sensor cell 120 detecting a NOx concentration, and a heater 103 generating heat in response to electric power supplied from a +B terminal of a battery source.
  • a NOx concentration sensing means M1 is connected to a sensor cell electrode of the gas concentration sensor 100 for applying a voltage to the sensor cell 120 to generate a current signal responsive to a detected NOx concentration and for sending the produced current signal to an external device.
  • An impedance sensing means M2 is also connected to the sensor cell electrode for detecting a current value corresponding to an impedance of the sensor cell 120 and for sending an actuation signal to the heater 103 so that the impedance of the sensor cell 120 is equalized to a predetermined target value.
  • An oxygen concentration sensing means M3 is connected to a pump cell electrode of the gas concentration sensor 100 for applying a voltage to the pump cell 110 to detect a current signal responsive to a detected oxygen concentration and for sending the produced current signal to an external device.
  • a heater actuating means M4 is provided for actuating the heater 103 in accordance with the signal produced from the impedance sensing means M2.
  • Fig. 3 shows an arrangement of the gas concentration sensor 100.
  • the gas concentration sensor 100 is a so-called combined or composite gas sensor having a double cell arrangement capable of simultaneously detecting both of the NOx concentration and the oxygen concentration.
  • the gas concentration sensor 100 comprises the pump cell 110, a porous diffusive layer 101, the sensor cell 120, and an atmospheric duct 102 and the heater 103, which are stacked or laminated integrally.
  • the gas concentration sensor 100 is installed to an engine exhaust pipe at its right end shown in Fig. 3. Thus, not only the upper and lower surfaces but the left surface of the sensor 100 are exposed to the exhaust gas.
  • the pump cell is brought into contact with the porous diffusive layer 101 at one side and is exposed to the exhaust gas at the other side.
  • a first pump electrode 111 provided on an upper surface of the pump cell 110, is exposed to the exhaust gas.
  • a second pump electrode 112 is interposed between a lower surface of the pump cell 110 and an upper surface of the porous diffusive layer 101.
  • the sensor cell 120 is located between the porous diffusive layer 101 and the atmospheric duct 102.
  • a first sensor electrode 121 provided on an upper surface of the sensor cell 120, contacts with the lower surface of the porous diffusive layer 101.
  • a second sensor electrode 122 provided on a lower surface of the sensor cell 120, faces to the atmospheric duct 102.
  • the exhaust gas introduced from the left end of the porous diffusive layer 101, flows in this porous diffusive layer 101 toward the right end thereof.
  • the solid electrolytic element is made of a sintered oxide material having appropriate oxygen ion conductivity.
  • the sintered oxide material comprises a base material, such as ZrO2, HfO2, ThO2, and Bi2O3, mixed with a solid soluble stabilizing material, such as CaO, MgO, Y2O3, and Yb2O3.
  • the porous diffusive layer 101 is a heat-resistant inorganic material such as alumina, magnesia, quartzite, spinel, and mullite.
  • a precious metal having adequate catalytic activity such as platinum Pt, is preferably used to form the first pump electrode 111 of the pump cell 110 and the first and second sensor electrodes 121 and 122 of the sensor cell 120.
  • the heater 103 is embedded in an insulation layer 104.
  • the atmospheric duct 102 serving as a reference gas chamber, is located between the insulation layer 104 and the sensor cell 120. External air, introduced into the atmospheric duct 102, serves as a reference gas providing a standard oxygen concentration.
  • the insulation layer 104 is formed by an alumina member or the like.
  • the heater 103 is formed by a cermet including platinum and alumina components. The heater 103 generates thermal energy in response to electric power supplied from an external power source to activate the sensor body including the pump cell 110 and the sensor cell 120 (as well as the electrodes).
  • the heater 103 has a flat and extended heat generating portion so that a uniform sensor temperature distribution can be realized by thoroughly heating the pump cell 110 and the sensor cell 120.
  • Figs. 4A to 4C explain the operation of the above-described gas concentration sensor 100.
  • exhaust gas components enter into the porous diffusive layer 101 from the left end.
  • the exhaust gas contains gas components of oxygen (O2), nitrogen oxides (NOx), carbon dioxide (CO2), and water (H2O).
  • the second pump electrode 112 of the pump cell 110 is formed by a NOx inactive electrode (which is unable to decompose the NOx gas). Accordingly, as shown in Fig. 4B, the pump cell 110 decomposes only the oxygen (O2) component from the exhaust gas. The decomposed oxygen component is discharged through the first pump electrode 111 into the exhaust gas. The current flowing in the pump cell 110 during the discharge phenomenon is detectable as an indicia representing an oxygen concentration of the exhaust gas.
  • the pump cell 110 cannot decompose all of the oxygen (O2) in the exhaust gas.
  • part of the undecomposed oxygen components continuously flows and will reach the sensor cell 120, where the residual oxygen (O2) and the NOx are decomposed by applying the voltage to the sensor cell 120 as shown in Fig. 4C.
  • both of O2 and NOx are decomposed on the first sensor electrode 121 of the sensor cell 120.
  • the decomposed components move across the sensor cell 120 and are discharged from the second sensor electrode 122 into the atmospheric duct 102.
  • the current flowing in the sensor cell 120 during the discharge phenomenon is detectable as an indicia representing a NOx concentration of the exhaust gas.
  • Fig. 5 shows the characteristics of the pump cell 110 in the oxygen concentration sensing operation.
  • Fig. 6 shows the characteristic of the sensor cell 120 in the NOx concentration sensing operation.
  • the pump cell has the limit-current characteristics in relation to the oxygen concentration.
  • the abscissa represents a voltage Vp applied to the pump cell, and the ordinate represents a current Ip flowing through the pump cell.
  • the limit current is sensible at a straight line region parallel to the V axis. The limit-current sensible region shifts toward the higher voltage side with increasing oxygen concentration.
  • the voltage control is performed so that the applied voltage increases with an inclination equivalent to that of a D.C. resistance component (i.e., an inclined portion increasing in proportion to an applied voltage).
  • the applied voltage is changed with reference to an application voltage line LX1 shown in Fig. 5, so that the desirable sensor current (i.e., limit current) can be detected regardless of the oxygen concentration in the exhaust gas.
  • the second pump electrode 112 of the pump cell 110 is a NOx inactive electrode, no NOx gas is decomposed at the pump cell 110.
  • the applied voltage exceeds a predetermined value, the NOx decomposition begins as shown in Fig. 5.
  • the obtainable pump cell current is responsive to both of the oxygen concentration and the NOx concentration (refer to each dotted line portion in Fig. 5). Accordingly, the application voltage line LX1 is set so as not to cross with the NOx decomposing regions (dotted line portions).
  • the sensor cell has the limit-current characteristics in relation to the NOx concentration.
  • the abscissa represents a voltage Vs applied to the sensor cell, and the ordinate represents a current Is flowing through the sensor cell.
  • A1 represents an offset current produced due to a residual oxygen flowing into the sensor cell 120 via the porous diffusive layer 101.
  • A2 represents a NOx decomposition current.
  • A3 represents a H2O decomposition current.
  • a current value "A1+A2" represents a limit current corresponding to the NOx concentration in the exhaust gas.
  • the limit-current sensible region defining the NOx decomposition current, is a straight-line portion parallel to the V axis.
  • the limit-current sensible region slightly shifts toward the higher voltage side with increasing NOx concentration.
  • the voltage control is performed so that the applied voltage is controlled so as to increase according to an application voltage line LX2 shown in Fig. 6.
  • the desirable sensor current i.e., limit current
  • Fig. 7A shows a relationship between a cell temperature (°C) and an A.C. impedance Zac ( ⁇ ) in a pump cell.
  • Fig. 7B shows a relationship between a cell temperature (°C) and an A.C. impedance Zac ( ⁇ ) in a sensor cell.
  • each cell shows a correlation between the cell temperature and the A.C. impedance.
  • Fig. 8 shows the temperature dependency of a sensor cell.
  • the sensor cell for detecting the NOx concentration has a temperature dependency.
  • the limit current sensible region i.e., flat region
  • the sensor output representing NOx concentration may vary largely and become unstable. Accordingly, to maintain the NOx concentration sensing accuracy at an appropriate level, it is inevitably necessary to accurately detect the sensor cell impedance and perform the heat control so as to equalize the impedance value to a constant value.
  • Fig. 2 shows a detailed arrangement of the gas concentration sensing apparatus shown in Fig. 1.
  • a lean gas is introduced into the porous diffusive layer 101 of the gas concentration sensor 100. Excessive oxygen is discharged from the porous diffusive layer 101 via the pump cell 110.
  • the flow direction of pump cell current Ip at this moment defines a positive terminal and a negative terminal of the pump cell 110.
  • the terminal connected to the first pump electrode 111 serves as a positive terminal.
  • the terminal connected to the second pump electrode 112 serves as a negative terminal.
  • the flow direction of sensor cell current Is at the moment the lean gas is introduced into the porous diffusive layer 101 defines the positive and negative terminals of the sensor cell 120.
  • the terminal connected to the second sensor electrode 122 serves as a positive terminal.
  • the terminal connected to the first sensor electrode 121 serves as a negative terminal.
  • a reference voltage circuit 201 and an amplification circuit 202 are connected to the common negative terminal of the pump cell 110 and the sensor cell 120.
  • the reference voltage circuit 201 produces a voltage Va which is supplied to a non-inverting input terminal of the amplification circuit 202.
  • An output terminal of the amplification circuit 202 is connected to an inverting input terminal of the amplification circuit 202 so as to constitute a voltage-follower arrangement.
  • the voltage Va of the reference voltage circuit 201 is applied to the second pump electrode 112 (i.e., negative terminal of pump cell 110) and to the first sensor electrode 121 (i.e., negative terminal of sensor cell 120). With this arrangement, an electric potential of the common negative terminal of respective cells 110 and 120 is maintained at the reference voltage Va higher than a GND voltage (0V).
  • a control circuit 200 is a microcomputer comprising a CPU and A/D and D/A converters.
  • A/D converters i.e., A/D0 to A/D3 receive the voltages of terminals Vc, Ve, Vd and Vb, respectively.
  • D/A converters i.e., D/A1 and D/A0
  • Another D/A converters i.e., D/A2 and D/A3 produce a NOx concentration signal and an oxygen concentration signal, respectively.
  • the pump command voltage Vb is supplied to the non-inverting input terminal of the amplification circuit 211.
  • An output terminal of the amplification circuit 211 is connected to one terminal of a current detecting resistor 212 which is provided to detect the pump cell current Ip responsive to the oxygen concentration.
  • the other terminal of the current detecting resistor 212 is connected to the first pump electrode 111 (i.e., positive pump cell terminal) of the gas concentration sensor 100, and is also connected to the inverting input terminal of the amplification circuit 211.
  • the converters A/D2 and A/D3 of the control circuit 200 are connected to the terminals of the current detecting resistor 212, respectively. With this arrangement, an electric potential of first pump electrode 111 is controlled to be always equalized to the pump command voltage Vb.
  • the current detecting resistor 212 in response to the pump command voltage Vb fed from the converter D/A1 to the pump cell 110, the current detecting resistor 212 has terminal voltage Vd and Vb at both ends thereof.
  • R1 represents a resistance value of the current detecting resistor 212
  • the pump cell current Ip (Vd ⁇ Vb) / R1
  • the control circuit 200, the amplification circuit 211,and the current detecting resistor 212 cooperatively function as the oxygen concentration sensing means M3 shown in Fig. 1.
  • the sensor command voltage Vc generated from the convertor D/A0 of the control circuit 200 is supplied to the non-inverting input terminal of an amplification circuit 221 via a LPF (i.e., low-pass filter) 230, such as a linear filter consisting of a resistor and a capacitor.
  • An output terminal of the amplification circuit 221 is connected to one terminal of a current detecting resistor 222 which is provided to detect the sensor cell current Is responsive to the NOx concentration.
  • the other terminal of the current detecting resistor 222 is connected to the second sensor electrode 122 (i.e., positive sensor cell terminal) of the gas concentration sensor 100, and is also connected to the inverting input terminal of the amplification circuit 221.
  • the converters A/D0 and A/D1 of the control circuit 200 are connected to the terminals of the current detecting resistor 222, respectively. With this arrangement, an electric potential of second sensor electrode 122 is controlled to be always equalized to the sensor command voltage Vc.
  • the current detecting resistor 222 in response to the sensor command voltage Vc fed from the converter D/A0 to the sensor cell 120, the current detecting resistor 222 has terminal voltage Ve and Vc at both ends thereof.
  • R2 represents a resistance value of the current detecting resistor 222
  • the sensor cell current Is (Ve ⁇ Vc) / R2
  • control circuit 200 the amplification circuit 221,and the current detecting resistor 222 cooperatively function as the NOx concentration sensing means M1 shown in Fig. 1.
  • the control circuit 200 detects an A.C. impedance of sensor cell 120 based on the sweep method. Namely, during the impedance detection of sensor cell 120, the control circuit 200 changes the voltage applied to the sensor cell 120 instantaneously through the converter D/A0. The applied voltage is modified into a sine wave form through LPF 230 and applied to the sensor cell 120. A desirable A.C. voltage frequency is 10 kHz or above. The time constant of LPF 230 is set to approximately 5 ⁇ s. The converters A/D1 and A/D0 detect the changes of terminal voltages Ve and Vc at the both ends of current detecting resistor 222, respectively. The A.C. impedance of sensor cell 120 can be calculated based on the voltage change amount and the current change amount thus obtained.
  • CPU In the control circuit 200, CPU generates a control command Duty through its I/O port to actuate a MOSFET driver 300.
  • the MOSFET driver 300 controls a MOSFET 310 so as to perform a PWM control in accordance with the control command Duty.
  • a controlled electric power is supplied from a power source 320 (e.g., a battery) to the heater 103.
  • Figs. 9 through 12 are flowcharts explaining the control processing performed in the control circuit 200.
  • Fig. 9 shows a main routine performed in the CPU of control circuit 200 at predetermined time intervals (e.g., 4 msec).
  • step 200 performs the application voltage control and the gas concentration sensing processing.
  • the voltage applied to the gas concentration sensor 100 is controlled in accordance with a later-described processing procedure shown in Fig. 10.
  • Step 300 checks whether an impedance detecting timing for the gas concentration sensor 100 has come or not. For example, when a predetermined time has passed from the previous impedance detection, the judgement result in step 300 becomes YES and the control flow proceeds to step 350.
  • An appropriate impedance detecting interval may be 128 msec in an engine startup condition and will be extendable to 256 msec in a stationary driving condition.
  • Step 350 performs the impedance detection in accordance with a later-described processing procedure shown in Fig. 11.
  • Step 400 checks whether a heater control timing for the gas concentration sensor 100 has come or not. For example, when a predetermined time (e.g., 128 msec) has passed from the previous heater control, the judgement result in step 400 becomes YES and the control flow proceeds to step 450 wherein the heater control is performed in accordance with a later-described processing procedure shown in Fig. 12.
  • a predetermined time e.g. 128 msec
  • step 200 The application voltage control shown in step 200 will be explained with reference to the flowchart shown in Fig. 10.
  • Steps 201 and 202 detect terminal voltages Vd and Vb at the terminals of the current detecting resistor 212 through the convertors A/D2 and A/D3, respectively.
  • steps 203 and 204 detect terminal voltages Ve and Vc at the terminals of the current detecting resistor 222 through the convertors A/D1 and A/D0, respectively.
  • Step 205 calculates the pump cell current Ip.
  • Step 206 obtains a target application voltage for obtaining the calculated pump cell current Ip with reference to the application voltage line LX1 shown in Fig.5. Namely, the target application voltage is obtained through a map calculation.
  • Step 207 outputs the obtained target application voltage through the converter D/A1 as the command voltage Vb.
  • step 208 calculates the sensor cell current Is.
  • Step 209 obtains a target application voltage for obtaining the calculated sensor cell current Is with reference to the application voltage line LX2 shown in Fig. 6. Namely, the target application voltage is obtained through a map calculation.
  • Step 210 outputs the obtained target application voltage through the converter D/A0 as the command voltage Vc.
  • step 211 outputs the calculated sensor cell current Is through the converter D/A2.
  • the sensor cell current Is is sent, as a current value representing the NOx concentration, to an engine control microcomputer or other external device.
  • step 212 outputs the calculated pump cell current Ip through the converter D/A3.
  • the pump cell current Ip is sent, as a current value representing the oxygen concentration, to the engine control microcomputer or other external device.
  • the serial communication will be used to perform the steps 211 and 212 for outputting the gas concentration signals.
  • steps 351 and 352 detect terminal voltages Ve and Vc at the terminals of the current detecting resistor 222 through the convertors A/D1 and A/D0, respectively.
  • the detected voltage values obtained in these steps 351 and 352 are referred to as current values Ve1 and Vc1 obtainable before the voltage changes.
  • step 353 outputs the voltage Vs+ ⁇ Vs through the converter D/A0, wherein Vs represents the present sensor cell application voltage and ⁇ Vs represents a predetermined A.C. voltage.
  • the applied voltages i.e., Vc and Ve terminal voltages
  • steps 354 and 355 detect terminal voltages Ve and Vc at the terminals of the current detecting resistor 222 through the convertors A/D1 and A/D0, respectively.
  • the detected voltage values obtained in these steps 354 and 355 are referred to as current values Ve2 and Vc2 obtainable after the voltage changes.
  • step 357 outputs ⁇ Vs2 through the converter D/A0.
  • the voltage ⁇ Vs2 is applied to the sensor cell 120 to return the application voltage to the original voltage value Vs.
  • step 451 checks whether the impedance feedback control condition is established or not.
  • the impedance Zac of sensor cell 120 is usable to check the establishment of the impedance feedback control condition.
  • the control flow proceeds to step 452 to perform the heater power control based on a 100% power supply.
  • a constant power control e.g., 40W
  • the PI (proportional-plus-integral) control can be used for performing the constant power control.
  • a proportional term GP and an integral term GI can be obtained in the following manner.
  • GP KP ⁇ (W ⁇ WT)
  • GI GI + KI ⁇ (W ⁇ WT)
  • KP represents a proportional constant
  • KI represents an integral constant
  • W represents a detected heater power
  • WT represents a target heater power.
  • the control command value Duty equal to "GP+GI", is used to control the heater power.
  • step 451 it is then checked in step 453 whether a predetermined time (e.g., 20 seconds) has passed or not.
  • a predetermined time e.g. 20 seconds
  • the control flow proceeds to step 454 to perform the impedance feedback control based on a PID control technique.
  • step 455 the control flow proceeds to step 455 to perform the impedance feedback control based on a PI control technique.
  • the PID control is temporarily performed to improve the temperature convergence at a final stage of the heating operation of the gas concentration sensor 100. Thereafter, the PI control begins. However, it will be possible to omit the steps 453 and 454 if the temperature convergence is acceptable.
  • the impedance feedback control (i.e., PID control) in the step 454 is performed in the following manner. More specifically, a proportional term GP, an integral term GI, and a differential term GD can be obtained in the following manner.
  • GP KP ⁇ (Zac ⁇ ZacT)
  • GI GI + KI ⁇ (Zac ⁇ ZacT)
  • GD KD ⁇ (Zac ⁇ ZacB)
  • Zac represents a detected impedance value of the sensor cell 120
  • ZacT represents a target impedance value of the sensor cell 120
  • ZacB represents a previously detected impedance value of the sensor cell 120.
  • the control command value Duty equal to "GP+GI+GD", is used to control the heater power.
  • the control command value Duty is used to control the heater power.
  • the above-described first embodiment has the following effects.
  • the heater power control is performed based on the impedance of the sensor cell 120 (i.e., second cell), the sensor cell 120 maintains a constant impedance. Accordingly, it becomes possible to prevent the NOx concentration sensing accuracy from being deteriorated due to undesirable temperature change of sensor cell 120 caused by the exhaust gas temperature change or the gas flow speed change. As a result, the purpose of the present invention, i.e., assuring the gas concentration sensing accuracy, can be attained. In this case, the temperature control of both cells 110 and 120 can be adequately performed by using only one heater 103.
  • the electric potential of the negative terminals of the pump cell 110 (i.e., first cell) and sensor cell 120 (i.e., second cell) are maintained at a level higher than 0V.
  • negative current flows in each cell.
  • the gas concentration in the porous diffusive layer 101 at a constant value regardless of lean or rich of the exhaust gas.
  • the oxygen concentration can be always maintained at a stoichiometric condition.
  • the present invention enables the rich gas detection, expands the sensible range of the gas concentration, and improves the response of a gas concentration signal in a transient condition from a rich gas to a lean gas.
  • Fig. 14 shows a modified arrangement of Fig. 2, according to which both the NOx concentration sensing means M1 and the oxygen concentration sensing means M3 are constituted by analog circuits for detecting analog gas concentration outputs.
  • Fig. 14 is different from Fig. 2 in that an amplification circuit 301 is provided for differential amplifying the terminal voltages of the current detecting resistor 212 and an amplification circuit 303 is provided for differential amplifying the terminal voltages of the current detecting resistor 222.
  • the amplification circuit 301 serving as an output section of the oxygen concentration sensing means M3, comprises an operational amplifier 301a and resistors 301b to 301e.
  • the amplification circuit 303 serving as an output section of the NOx concentration sensing means M1, comprises an operational amplifier 303a and resistors 303b to 303e.
  • the arrangement of Fig. 14 is advantageous in that the gas concentration sensing signal is continuous and, therefore, the gas concentration sensing signal can be obtained speedily.
  • the gas concentration sensor 100 comprises a plurality of cells 110 and 120
  • respective cells have cell temperatures different from each other.
  • the overall sensor temperature distribution may not be ignorable.
  • the gas concentration sensor 100 has an extended heat-generating portion so as to realize a uniform heating distribution.
  • the sensor cell temperature is slightly lower than the pump cell temperature due to heat release or loss at the sensor installation portion near the sensor cell. For example, even when the engine is in an idling condition or when the exhaust gas temperature or the gas flow speed is constant, the temperature near the sensor cell electrode is approximately 760°C while the temperature near the pump cell electrode is approximately 800°C, causing a temperature difference of approximately 40 °C.
  • the pump cell temperature increase is faster than the sensor cell temperature increase.
  • the temperature difference between the pump cell 110 and the sensor cell 120 becomes larger compared with the equilibrium condition. If the feedback control is performed in this condition for equalizing the sensor cell impedance to a target value, the pump cell temperature will excessively increase due to an overshoot shown in Fig. 15. The excessive temperature increase possibly deteriorates the pump cell 110.
  • a 100% power control or a constant power control is performed before time t1. Then, an impedance feedback control is performed after the time t1 to equalize the sensor cell impedance to a constant value.
  • the pump cell temperature tends to increase or decrease excessively because the pump cell 110 is exposed to the exhaust gas. This possibly changes the temperature distribution of the gas concentration sensor 100 and worsens the function of the gas concentration sensor 100.
  • the second embodiment detects a pump cell impedance in addition to the sensor cell impedance.
  • Zs represents the impedance of sensor cell 120
  • Zp represents the impedance of pump cell 110.
  • the heater power is controlled by selectively performing a feedback control based on a detected Zs value and a feedback control based on a detected Zp value.
  • Fig. 16 shows an arrangement of a gas concentration sensing apparatus in accordance with the second embodiment.
  • the arrangement shown in Fig. 16 is different from the arrangement shown in Fig. 2 in that an LPF 240 is added for smoothing or relaxing an A.C. change of the applied voltage in the detection of the pump cell impedance.
  • the control circuit 200 detects the impedance of pump cell 110 based on the voltage values Vd and Vb detected via the converters A/D2 and A/D3 in addition to the impedance detection of the sensor cell 120.
  • control circuit 200 instantaneously changes the pump cell voltage which is outputted via the converter D/A1.
  • the pump cell voltage is then modified into a sine wave form through the LPF 240 and applied to the pump cell 110.
  • a desirable A.C. voltage frequency is 10 kHz or above.
  • the time constant of LPF 240 is set to approximately 5 ⁇ s.
  • the converters A/D2 and A/D3 detect the changes of terminal voltages Vd and Vb at the both ends of current detecting resistor 212, respectively.
  • the A.C. impedance of pump cell 110 can be calculated based on the voltage change amount and the current change amount thus obtained.
  • the impedance detecting procedure for the pump cell 110 is fundamentally identical with the processing procedure shown in Fig. 11 and, therefore, will not be disclosed and explained specially.
  • the terminal voltages Vd and Vb of the current detecting resistor 212 are detected through the convertors A/D2 and A/D3, respectively.
  • the impedance Zp of pump cell 110 is calculated based on the voltage values thus detected. It is preferable to execute the above-described Zp value detecting processing immediately after the step 230, i.e., the sensor cell impedance detecting processing, in the main routine shown in Fig. 9.
  • Figs. 17 and 18 are a flowchart showing a heater control subroutine in accordance with the second embodiment. In other words, this routine is replaced for the routine shown in Fig. 12.
  • the heater control procedure shown in Figs. 17 and 18 will be explained with reference to time charts shown in Fig. 19 and 20.
  • step 501 checks whether a sensor cell feedback flag FSS is 1 or not.
  • the sensor cell feedback flag FSS indicates that the impedance feedback control for the sensor cell 120 is presently performed. Thus, FSS is "0" in a condition (i.e., in an initial condition) where the feedback control is not executed. FSS is turned to "1" after the feedback control is started.
  • step 503 the control flow proceeds to step 505 to check whether a pump cell feedback flag FPS is 1 or not.
  • the pump cell feedback flag FPS indicates the execution of the pump cell impedance feedback control in the temperature increasing process starting from the cold startup condition.
  • step 507 checks whether a predetermined time (e.g., 30 seconds) has passed after the FPS is turned to 1.
  • a predetermined time e.g. 30 seconds
  • the step 508 executes the impedance feedback control for the pump cell 110. Namely, the heater power control is performed based on a PI or PID control technique.
  • the control flow proceeds to a later-described step 518 shown in Fig. 18.
  • the judgement result is YES in the step 507
  • the control flow proceeds to step 509 to set the sensor cell feedback flag FSS to 1.
  • the control flow proceeds to step 510 shown in Fig. 18.
  • the control flow directly proceeds to the step 510.
  • the judgement result is NO in the step 502
  • the control flow proceeds to the step 509 to set the sensor cell feedback flag FSS to 1. Then, the control flow proceeds to the step 510 of Fig. 18.
  • the temperatures of cells 110 and 120 continuously increase after starting the engine operation in a cold condition.
  • the pump cell temperature increases to 650 °C at the time t11.
  • the pump cell feedback flag FPS is turned to 1 and the impedance feedback control for the pump cell 110 is started (refer to steps 506 and 508 in Fig. 17).
  • the pump cell temperature reaches the target value under the impedance feedback control for the pump cell 110.
  • the sensor cell feedback flag FSS is turned to 1 (refer to step 509 in Fig. 17).
  • Tx temperature difference between the pump cell temperature TPS and the sensor cell temperature TSS
  • the pump cell temperature TPS and the sensor cell temperature TSS are indirectly obtainable from the corresponding cell impedances Zp and Zs, respectively.
  • step 516 When the judgement result is YES in the step 512(i.e., Tx ⁇ 70°), the control flow proceeds to step 516.
  • the step 516 performs the impedance feedback control of the sensor cell 120.
  • the judgement result is NO in the steps 511 and 512 (i.e., 70°C ⁇ Tx ⁇ 90°C)
  • the control flow proceeds to step 513 to selectively perform the steps 514 and 516 in accordance with the temperature difference flag FHi.
  • the temperature difference flag FHi is held to 0 (initial value) until the temperature difference Tx reaches 90°C (i.e., before the time t21). In this condition, the impedance feedback control for the sensor cell 120 is performed (refer to step 516 in Fig. 18). When the temperature difference Tx has just reached 90°C (i.e., at the time t21), the temperature difference flag FHi is turned to 1. The impedance feedback control for the pump cell 110 is performed (refer to step 514 in Fig. 18).
  • the temperature difference flag FHi is turned to 0 to resume the impedance feedback control for the sensor cell 120 (refer to step 516 in Fig. 18).
  • the temperature difference Tx is relatively small.
  • the impedance feedback control for the sensor cell 120 is performed.
  • the exhaust gas temperature may increase temporarily.
  • the pump cell temperature is sensitively influenced by the increased exhaust gas temperature.
  • the temperature difference Tx becomes relatively large.
  • the impedance feedback control for the pump cell 110 is performed. According to the processing procedure of the steps 511 to 517, a hysteresis is provided in the judgement of the temperature difference Tx. This is effective to eliminate the hunting phenomenon in the feedback control.
  • step 518 checks whether the pump cell temperature TPS is lower than a predetermined upper limit (e.g., 900°C).
  • the pump cell temperature TPS is indirectly obtainable from the pump cell impedance Zp.
  • the control flow proceeds to step 519.
  • the step 519 further checks whether the pump cell temperature TPS is higher than a predetermined lower limit (e.g., 650°C).
  • the control flow proceeds step 520.
  • Step 520 resets a sensor inactive flag FN to 0.
  • the control flow proceeds step 521.
  • step 522 When the judgement result is NO (i.e., TPS ⁇ upper limit), the control flow proceeds to step 522.
  • the step 522 forcibly stops the heater power supply to prevent the heater from being thermally damaged.
  • step 523 checks whether the sensor cell temperature TSS is higher than a predetermined lower limit (e.g., 650°C).The sensor cell temperature TSS is indirectly obtainable from the sensor cell impedance Zs.
  • a predetermined lower limit e.g., 650°C
  • the control flow proceeds step 524.
  • Step 524 resets the sensor inactive flag FN to 0.
  • the control flow proceeds step 525.
  • Step 525 sets the sensor inactive flag FN to 1. Needless to say, the above-described judgements in the steps 518, 519, and 523 can be performed by using the impedance Zp or Zs.
  • the inclination representing the D.C. resistance component varies in response to the temperature distribution change caused by the increased exhaust gas temperature. More specifically, as shown in Fig. 21, in the V-I characteristics, the inclination representing the D.C. resistance component increases with increasing pump cell temperature (refer to (A)) and decreases with decreasing pump cell temperature (refer to (B)).
  • the limit-current sensible region is slightly ascendent right relative to the V axis.
  • the voltage application point for determining the pump cell voltage is usually set to a center of the limit-current sensible region. Thus, the voltage application point shifts left in the inclination change of line (A), and shifts right in the inclination change of line (B).
  • the present invention provides new application voltage lines LXa, LXb and LXc in addition to the standard or basic application voltage line LX1.
  • One of the plurality of application voltage lines LX1,LXa, LXb and LXc is selectively used in accordance with the impedance value of the pump cell 110.
  • the pump cell temperature is high, i.e., when the inclination representing the D.C. resistance component increases, it is preferable to select the application voltage line LXa.
  • the pump cell temperature is low, i.e., when the inclination representing the D.C. resistance component decreases, it is preferable to select either the application voltage line LXb or LXc.
  • Such a selection of an optimum application voltage line from a plurality of application voltage lines makes it possible to appropriately set the application voltage even when the voltage application point for determining the pump cell voltage shifts right or left.
  • Fig. 23 is a flowchart showing a processing procedure for varying the pump cell application voltage by selectively using the plurality of application voltage lines LX1, LXa, LXb, and LXc shown in Fig. 22. For example, this routine can be inserted between the steps 204 and 205 of Fig. 10.
  • step 601 checks whether the pump cell temperature TPS is lower than a first reference value (e.g., 820°C).
  • step 602 checks whether the pump cell temperature TPS is lower than a second reference value (e.g., 780°C).
  • step 603 checks whether the pump cell temperature TPS is lower than a third reference value (e.g., 730°C).
  • the pump cell temperature TPS is indirectly obtainable from the pump cell impedance Zp.
  • step 601 When the judgement result is NO in the step 601, the control flow proceeds to step 604 to select the application voltage line LXa of Fig. 22.
  • step 602 When the judgement result is NO in the step 602, the control flow proceeds to step 605 to select the application voltage line LX1 of Fig. 22.
  • step 603 the control flow proceeds to step 606 to select the application voltage line LXb of Fig. 22.
  • step 603 When the judgement result is YES in the step 603, the control flow proceeds to step 607 to select the application voltage line LXc of Fig. 22.
  • the application voltage line LXa is selected when TPS ⁇ 820°C.
  • the application voltage line LX1 is selected when 780°C ⁇ TPS ⁇ 820°C.
  • the application voltage line LXb is selected when 730°C ⁇ TPS ⁇ 780°C.
  • the application voltage line LXc is selected when TPS ⁇ 730°C.
  • the steps 503, 511 to 513 of Figs. 17 and 18 cooperatively serve as a judging means of the present invention.
  • the steps 508, 514 and 516 of Figs. 17 and 18 cooperatively serve as a power control means of the present invention.
  • the above-described second embodiment makes it possible to adequately maintain the NOx concentration sensing accuracy regardless of the exhaust gas temperature or the gas flow speed change in the same manner as in the above-described first embodiment. Furthermore, the second embodiment brings the following effects. Namely, the second embodiment judges temperature conditions relating to the activation of respective cells 110 and 120 (refer to Figs. 17 and 18). Then, the second embodiment selectively performs the impedance feedback control of the pump cell 110 and the impedance feedback control of the sensor cell 120 in accordance with the judgement result. Thus, the gas concentration sensing accuracy is not deteriorated even when the temperature characteristics of each cell is differentiated due to the sensor structure. As a result, it becomes possible to adequately maintain the gas concentration sensing accuracy regardless of the fluctuation in the cell temperature distribution.
  • the impedance feedback control is performed for the high-temperature cell among the plurality of cells 110 and 120.
  • the impedance feedback control for the pump cell 110 begins early.
  • the impedance feedback control for the sensor cell 120 starts later.
  • the impedance feedback control for the high-temperature cell is performed when the temperature difference between cells is relatively large.
  • the impedance feedback control for the pump cell 110 is performed when the cell temperature difference is relatively large.
  • the impedance feedback control for the sensor cell 120 is performed when the cell temperature difference is relatively small. Accordingly, when the exhaust gas temperature is stable, the impedance feedback control for the sensor cell 120 is performed.
  • the impedance feedback control is performed for the cell which is most sensitively influenced by the exhaust gas temperature increase. With the above arrangement, it becomes possible to adequately maintain the gas concentration sensing accuracy regardless of the fluctuation in the cell temperature distribution.
  • the second embodiment guards the pump cell temperature and the sensor cell temperature by providing the predetermined upper and lower values.
  • the second embodiment controls the voltage applied to the pump cell 110 in accordance with the impedance of this pump cell 110. This is advantageous in that the pump cell voltage can be adequately managed and the oxygen concentration accuracy can be appropriately maintained. The improvement in the oxygen concentration sensing accuracy brings the improvement in the NOx concentration sensing accuracy.
  • the voltage applied to each cell for detecting the impedance has an A.C. component.
  • the A.C. current change of one cell may give adverse influence to the other cell whose impedance is not detected.
  • the pump cell impedance when the pump cell impedance is detected, a current flows in the sensor cell in synchronism with the application of an A.C. voltage to the pump cell.
  • the sensor cell current Is varies unwantedly (by several ⁇ A).
  • the pump cell current Ip varies unwantedly (by several ⁇ A).
  • the third embodiment of the present invention eliminates such an adverse interference during the impedance detection by providing a sample hold circuit in an output path of the oxygen concentration or NOx concentration signal.
  • Fig. 24 shows a detailed arrangement of a gas concentration sensing apparatus in accordance with the third embodiment.
  • Fig. 24 is different from Fig. 16 in that the amplification circuit 301, consisting of the operational amplifier 301a and the resistors 301b to 301e, is connected to both ends of the current detecting resistor 212, and also in that a sample hold circuit 302 is connected to the output terminal of the amplification circuit 301.
  • the sample hold circuit 302 samples the pump cell current Ip representing a detected oxygen concentration, and successively renews the sample value in a predetermined gate-on period during which a signal SG1 is turned on.
  • the amplification circuit 303 consisting of the operational amplifier 303a and the resistors 303b to 303e, is connected to both ends of the current detecting resistor 222.
  • a sample hold circuit 304 is connected to the output terminal of the amplification circuit 303. The sample hold circuit 304 samples the sensor cell current Is representing a detected NOx concentration, and successively renews the sample value in a predetermined gate-on period during which a signal SG2 is turned on.
  • the control circuit 200 produces a command signal Vb as a D/A1 signal to detect the pump cell impedance.
  • the control circuit 200 produces a SG2 signal to hold the sample hold circuit 304 provided for the sensor cell 120.
  • the NOx concentration signal corresponding to the sensor cell current Is is held at a value detected immediately before the gate off timing. In other words, it becomes possible to prevent the sensor output from fluctuating due to the cell interference.
  • control circuit 200 produces a command signal Vc as a D/A0 signal to detect the sensor cell impedance.
  • the control circuit 200 produces a SG1 signal to hold the sample hold circuit 302 provided for the pump cell 110.
  • the oxygen concentration signal corresponding to the pump cell current Ip is held at a value detected immediately before the gate off timing.
  • the gas concentration signal is held at a value immediately before starting the impedance detection. This makes it possible to prevent any sensor output (any gas concentration) from fluctuating.
  • the above-described third embodiment newly brings the following effects. Namely, according to the system temporarily changing the voltage applied to each cell for detecting the impedance of the pump cell 110 or the sensor cell 120, the current interference responsive to the voltage change occurs between the cells. Thus, the gas concentration signal may vary unwantedly. To solve such problems, the third embodiment provides the sample hold circuits 302 and 304 to hold the gas concentration signals at the latest values during the impedance detection.
  • the fourth embodiment detects both the pump cell impedance and the sensor cell impedance and performs the impedance feedback control of heater 103 based on a sum (or an average) of the detected impedance values.
  • the sensor cell temperature is controlled to a target value (i.e., 760°C)
  • the temperature distribution of gas concentration sensor 100 may vary.
  • the pump cell temperature increases to 910°C which is 110°C higher than the target value, the cell temperature difference becomes "+150°C".
  • the pump cell 110 is in an excessively heated condition.
  • the heater power control is performed in such a manner that the sum of cell temperatures is equalized to the target value (1,560°C). In each cell, the temperature decreases by an amount of 55°C.
  • the sensor cell temperature becomes 705°C and the pump cell temperature becomes 855°C.
  • the excessively heated condition of the pump cell 110 is eliminated through such a heater control.
  • the temperature distribution of gas concentration sensor 100 may vary oppositely.
  • the pump cell temperature decreases to 610°C which is 190°C lower than the target value.
  • the cell temperature difference becomes " ⁇ 150°C".
  • the pump cell 110 is in an inactive condition and therefore cannot operate accurately.
  • the heater power control is performed in such a manner that the sum of cell temperatures is equalized to the target value (1,560° C). In each cell, the temperature increases by an amount of 95°C.
  • the sensor cell temperature becomes 855°C and the pump cell temperature becomes 705°C.
  • the inactive condition of the pump cell 110 is eliminated through this heater control.
  • the application voltage is controlled in accordance with the impedance of each cell.
  • the application voltage line L11 is selected when the sensor cell temperature is 800°C.
  • the application voltage line L12 is selected when the sensor cell temperature is 750°C.
  • the application voltage line L13 is selected when the sensor cell temperature is 700°C. The voltage applied to the sensor cell 120 is controlled based on the selected application voltage line.
  • the fourth embodiment effectively eliminates the temperature difference when the temperature distribution varies due to temperatures of the cells 110 and 120. As a result, the fourth embodiment makes it possible to suppress the excessive temperature increase or decrease to the inactive temperature in respective cells 110 and 120, thereby realizing a stable gas concentration detection.
  • the fifth embodiment proposes a method for preventing the gas concentration sensing accuracy from worsening due to the impedance detection when the application voltage to the cells 110 and 120 is changed in the apparatus for detecting the pump cell impedance and the sensor cell impedance.
  • the command voltage i.e., application voltage
  • the LPF e.g., 230 or 240 shown in Fig. 16
  • the time constant of the LPF is set to a value suitable for the impedance detection. If this arrangement is directly used to detect a gas concentration, a large sensing error may be caused.
  • Fig. 27 shows why the gas concentration sensing error is caused.
  • the command voltage supplied to each cell varies stepwise as shown by a dotted line in Fig. 27.
  • the voltage signal varies with a waveform smoothly curving at each stepwise edge of the dotted line as shown by a solid line.
  • a detected limit current value possibly includes an error.
  • the limit current detected immediately after switching the application voltage value shown by ⁇ in Fig. 27
  • the limit current detected at an intermediate timing shown by ⁇ in Fig. 27
  • the fifth embodiment switches the LPF time constant.
  • a time constant used for the gas concentration detection is larger than a time constant for the impedance detection. This switching operation reduces the gas concentration sensing error shown in Fig. 27.
  • Increasing the LPF time constant means that the frequency is reduced in the change of the voltage applied to the gas concentration sensor 100. Namely, the A.C. impedance component becomes large, and the current change responsive to the application voltage change becomes small. Therefore, the deviation of the limit current from the true value becomes small as shown in Fig. 28. The gas concentration sensing error can be reduced.
  • the fifth embodiment selectively uses different time constants for the impedance detection and the gas concentration detection in each of LPF 230 and LPF 240 shown in Figs. 2 and 16.
  • Fig. 29 shows a practical arrangement of the LPF 230 and LPF 240.
  • a switch SW is provided to selectively use a resistor R1 or a resistor R2 (R1>R2) so as to change the time constant.
  • R1 or R2 R1>R2
  • the fifth embodiment reduces the gas concentration sensing error.
  • the sensing accuracy can be improved.
  • LPF 230 and 240 serve as a speed limiting means for limiting the change speed of the application voltage.
  • the control circuit 200 (CPU) can be used to limit the change speed of the application voltage.
  • An ordinary change speed is, for example, 5 mV/4 ms. It is desirable to delay this speed to 5 mV/30 ms.
  • the present invention can be variously modified.
  • the pump cell impedance it is possible to detect both the pump cell impedance and the sensor cell impedance.
  • the feedback control for the heater 103 is performed based on a detected sensor cell impedance value.
  • the pump cell voltage is controlled based on a detected pump cell impedance value. According to this modification, it becomes possible to adequately manage the pump cell voltage so as to assure an oxygen concentration sensing accuracy, in addition to the effects of the first embodiment.
  • the apparatus shown in Fig. 16 comprises two D/A converters and two low-pass filters for detecting both the pump cell impedance and the sensor cell impedance.
  • the command signal is produced from the converter D/A0 and sent to the LPF 260.
  • the D/A0 output includes an A.C. component to detect the impedance of each cell.
  • the impedance is obtainable based on a detected voltage change and a detected current change. According to this modification, it becomes possible to simplify the circuit arrangement because this arrangement requires only one D/A converter and only one LPF.
  • Fig. 31 shows a modified arrangement of Fig.30.
  • An application circuit 311 and a sample hold circuit 312 are connected to the terminal Vd of the current detecting resistor 212.
  • An application circuit 313 and a sample hold circuit 314 are connected to the terminal Ve of the current detecting resistor 222.
  • the sample hold circuits 312 and 313 hold the latest output values detected immediately before starting the impedance detection of each cell. It becomes possible to suppress the gas concentration signal from fluctuating unwantedly. Thus, the signal output is stabilized. More specifically, a latest oxygen concentration signal is held during the pump cell impedance detection. A latest NOx concentration signal is held during the sensor cell impedance detection.
  • the arrangement shown in Fig. 31 changes the voltage applied to the common terminal. Thus, it becomes possible to eliminate the interference between cell currents during the impedance detection.
  • the common negative terminal of the pump cell 110 and the sensor cell 120 i.e., second pump electrode 112, first sensor electrode 121 is maintained at the reference voltage Va higher than the ground potential (0V). It is however possible to modify this arrangement in the following manner. For example, only one negative terminal of the two cells (e.g., the terminal of pump cell 110 shown in Fig. 1) can be directly connected to the ground while the other negative terminal is maintained at the reference voltage level. Alternatively, it is possible to connect both of the negative terminal of the two cells to the ground.
  • the above-described embodiment provides a total of four application voltage lines as shown in Fig. 22 to selectively use one of them based on a detected pump cell impedance. It is however possible to modify this arrangement in the following manner. For example, the total number of application voltage lines selectable in the V-I characteristics shown in Fig. 22 can be reduced to three or less or can be increased to five or more. Furthermore, it is possible to prepare only one (i.e., standard) application voltage line LX1 and correct the inclination of the line LX1 in accordance with a detected pump cell impedance.
  • the above-described embodiment performs the heater power control based on the target value equal to a sum of the pump cell impedance and the sensor cell impedance. It is however possible to perform the heater power control based on an average of the pump cell impedance and the sensor cell impedance. The same effects will be obtained.
  • the above-described embodiments temporarily vary the cell voltage based on the sweep method.
  • An A.C. impedance is measurable from a detected current change. It is however possible to use other methods. For example, it is possible to temporarily vary the cell current during the resistance detection and obtain an A.C. impedance value from a detected voltage change.
  • Fig. 32 shows a practical arrangement as a partly-modified embodiment of Fig. 2.
  • Fig. 32 is different from Fig. 2 in that a switch SW1 is provided between the second sensor electrode 122 of the gas concentration sensor 100 and the current detecting resistor 222.
  • the switch SW1 has a contact "a” connected to a constant current source 350 which produces a constant current Icst and another contact “b” connected to the current detecting resistor 222.
  • the switch SW1 is usually connected to the contact "b.”
  • the control circuit 200 causes the switch SW1 to the contact "a" to temporarily supply the constant current Icst to the sensor cell 120.
  • the voltage Vp is given to the second sensor electrode 122.
  • the switch SW1 is switched to the contact "a" to supply the constant current Icst to the sensor cell 120.
  • a voltage Vp detectable at this moment is measured through the converter A/D0.
  • the detected value is referred to as AD2.
  • Impedance ⁇ V / Icst
  • the switch SW1 is returned to the contact "b."
  • the impedance detection of the pump cell 110 it can be done in the same manner as in the above-described sensor cell impedance detection.
  • a D.C. element resistance is measured as the internal resistance of each cell.
  • the heater feedback control or the application voltage control is performed based on a detected D.C. element resistance value.
  • the D.C. element resistance is detectable in the following manner.
  • a voltage e.g., a negative voltage
  • the D.C. element resistance is based on a detected current value (a negative current value).
  • the present invention is not limited to the above-described gas concentration sensor 100 and therefore can be applied, for example, to a gas concentration sensor shown in Fig. 34.
  • the gas concentration sensor 150 shown in Fig. 34 is different from the gas concentration sensor 100 shown in Fig. 3 in that the pump cell and the sensor cell are oppositely arranged.
  • a pump cell 160 detecting an oxygen concentration in the exhaust gas is interposed between the porous diffusive layer 101 and the atmospheric duct 102.
  • a sensor cell 170 detecting a NOx concentration in the exhaust gas is laminated or stacked on the porous diffusive layer 101 at one side and is exposed to the exhaust gas at the other side.
  • the pump cell 160 has first and second pump electrodes 161 and 162.
  • the sensor cell 170 has first and second sensor electrodes 171 and 172.
  • the first pump electrode 161, facing the porous diffusive layer 101, is made by a precious metal inactive to the NOx gas, such as Au-Pt.
  • the first pump electrode 161 is an electrode which is unable to decompose the NOx gas.
  • Other electrodes are made of noble metals having high catalytic activity, such as platinum.
  • the gas concentration sensor 150 (cells 160 and 170) is similar to the above-described gas concentration sensor 100 in the gas concentration sensing principle and in the sensor signal characteristics which are already explained and will not be explained again.
  • the present invention is not limited to the double-cell type gas concentration sensors explained in the above embodiments.
  • the present invention can be applied to other gas concentration sensors which may have a triple-cell arrangement or comprises four or more cells consisting of divided pump cells and divided sensor cells.
  • gas concentration sensing apparatus having a triple-cell arrangement will be explained.
  • Fig. 35 shows a triple-cell type gas concentration sensor 400 which chiefly comprises a pump cell 410 (i.e., first cell), a reference cell 430, a sensor cell 420 (i.e., second cell), and a heater 440.
  • the pump cell 410 discharges the oxygen contained in the exhaust gas to detect the oxygen concentration.
  • the reference cell 430 detects a partial pressure of the oxygen.
  • the sensor cell 420 decomposes the NOx gas and discharges the oxygen ions to detect the NOx concentration.
  • the exhaust gas emitted from the engine is introduced into a first chamber405 via a porous diffusive layer 401.
  • the pump cell 410 discharges the oxygen from the first chamber 405 without decomposing the NOx based on a monitored voltage value of the reference cell 430.
  • the voltage of reference cell 430 can be monitored based on a voltage difference between a first reference electrode 431 and a second reference electrode 432. Namely, the oxygen concentration is measured based on a current flowing in response to a voltage applied between a first pump electrode 411 and a second pump electrode 412.
  • the residual exhaust gas is introduced into a second chamber 406 via a second porous diffusive layer 404.
  • the NOx gas residing in the second chamber 406 is decomposed on the sensor cell 420 and discharged. Namely, the NOx gas is discharged by applying a voltage applied between a first sensor electrode 421 and a second sensor electrode 422.
  • the NOx concentration can be detected by measuring a current flowing in response to the voltage applied between the first sensor electrode 421 and the second sensor electrode 422.
  • the electromotive force of the reference cell 430 reduces and the voltage of second reference electrode 432 reduces.
  • the oxygen in the first chamber 405 is discharged to the exhaust gas side (i.e., the upper side of Fig. 35) via the pump cell 410.
  • the electromotive force of the reference cell 430 increases and the voltage of second reference electrode 432 increases. In this case, an oxygen amount discharged from the first chamber 405 decreases.
  • the oxygen concentration in the exhaust gas can be detected by measuring the pump cell current detectable at this moment.
  • the exhaust gas is introduced into the second chamber 406 via the second porous diffusive layer 404.
  • the sensor cell 420 decomposes the NOx gas and discharges the oxygen ions to an atmospheric duct 407 via the second chamber 406.
  • the NOx concentration in the exhaust gas can be detected by measuring the sensor cell current detectable at this moment.
  • the heater 440 is embedded in insulation layers 441 and 442. When electric power is supplied to the heater 440, the heater 440 produces heat energy so as to activate an entire sensor body including the cells 410, 420 and 430 (as well as the electrodes).
  • the heater power control can be performed based on the impedance of sensor cell 420 (i.e., second cell) so as to equalize the sensor cell impedance to a constant value as explained in the first embodiment. Accordingly, the sensor cell temperature does not fluctuate due to the exhaust gas temperature change or gas flow speed change. The NOx concentration sensing accuracy does not deteriorates. Thus, it becomes possible to attain the object of the present invention. Namely, the gas concentration sensing accuracy is adequately maintained.
  • the gas concentration sensing accuracy does not deteriorate even when respective cells have temperature characteristics different from each other due to their structures. As a result, it becomes possible to adequately maintain the gas concentration sensing accuracy regardless of the fluctuation in the cell temperature distribution.
  • the impedance feedback control is performed for the high-temperature cell among the plurality of cells 410 to 430. Subsequently, the impedance feedback control for the sensor cell 420 starts. Furthermore, when the temperature difference between cells is relatively large, the impedance feedback control is performed for the highest-temperature cell. The impedance feedback control for the sensor cell 420 is performed when the cell temperature difference is relatively small. Accordingly, when the exhaust gas temperature is stable, the impedance feedback control for the sensor cell 420 is performed. When the exhaust gas temperature increases temporarily, the impedance feedback control is performed for the cell which is most sensitively influenced by the exhaust gas temperature increase. With the above arrangement, it becomes possible to adequately maintain the gas concentration sensing accuracy regardless of the fluctuation in the cell temperature distribution.
  • the present invention is not limited to a gas concentration sensor capable of detecting both the oxygen concentration and the NOx concentration.
  • the present invention is applicable to a gas concentration sensor capable of detecting both the oxygen concentration and a HC or CO concentration.
  • the pump cell discharges excessive oxygen contained in the exhaust gas (i.e., in the measuring gas) and then the sensor cell decomposes HC or CO contained in the residual exhaust gas. Accordingly, it becomes possible to detect the HC or CO concentration in addition to the oxygen concentration.
  • a gas concentration sensor (100) comprises a pump cell (110) for detecting an oxygen concentration in an exhaust gas and a sensor cell (120) for detecting a NOx concentration in the exhaust gas.
  • a porous diffusive layer (101) is interposed between these cells (110, 120).
  • the sensor (100) comprises a heater (103) for heating these cells (110, 120).
  • a control circuit (200) produces a sensor cell voltage having an A.C. component to detect the impedance of sensor cell (120). The electric power supplied to the heater (103) is controlled based on a detected impedance value of the sensor cell (120).

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  • Chemical & Material Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Biochemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Electrochemistry (AREA)
  • Physics & Mathematics (AREA)
  • Analytical Chemistry (AREA)
  • Molecular Biology (AREA)
  • General Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
  • Combined Controls Of Internal Combustion Engines (AREA)
  • Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)
  • Measuring Oxygen Concentration In Cells (AREA)
EP99118691A 1998-09-29 1999-09-22 Apparat zum Messen von Gaskonzentrationen Ceased EP0995986A3 (de)

Priority Applications (1)

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JP27552198 1998-09-29
JP27552198 1998-09-29
JP20492799 1999-07-19
JP20492799A JP3983422B2 (ja) 1998-09-29 1999-07-19 ガス濃度検出装置

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FR2825469A1 (fr) * 2001-05-31 2002-12-06 Denso Corp Systeme de controle d'alimentation electrique pour dispositif de chauffage utilise dans un capteur de gaz
EP1331478A1 (de) * 2002-01-24 2003-07-30 Volkswagen AG Verfahren zur Bestimmung der NOx-Konzentration in Abgasen
EP1202048A3 (de) * 2000-10-31 2004-06-02 Denso Corporation Gaskonzentration-Messvorrichtung mit Kompensation einer Fehlerkomponente des Ausgangssignals
WO2007099647A1 (en) * 2006-02-28 2007-09-07 Toyota Jidosha Kabushiki Kaisha Temperature control apparatus for heater-equipped sensor
WO2009082030A1 (en) * 2007-12-26 2009-07-02 Toyota Jidosha Kabushiki Kaisha Gas concentration detection apparatus

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JP4840529B2 (ja) * 2000-10-31 2011-12-21 株式会社デンソー ガス濃度検出装置
JP4524910B2 (ja) * 2000-12-08 2010-08-18 株式会社デンソー 積層型ガスセンサ及びそれを用いたガス濃度検出装置
JP4016790B2 (ja) 2002-10-10 2007-12-05 株式会社デンソー ガス濃度検出装置
JP2005055395A (ja) * 2003-08-07 2005-03-03 Denso Corp ガス濃度センサのヒータ制御装置
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JP4270286B2 (ja) * 2007-02-07 2009-05-27 トヨタ自動車株式会社 ガスセンサ用の制御装置
JP4870611B2 (ja) * 2007-04-19 2012-02-08 日本特殊陶業株式会社 ガスセンサ制御装置
JP4913659B2 (ja) * 2007-04-19 2012-04-11 日本特殊陶業株式会社 ガスセンサ制御装置
JP5030177B2 (ja) * 2007-11-05 2012-09-19 日本特殊陶業株式会社 ガスセンサ制御装置およびガスセンサ制御システム
JP5007666B2 (ja) * 2007-12-26 2012-08-22 トヨタ自動車株式会社 ガス濃度検出装置
JP5007689B2 (ja) * 2008-03-04 2012-08-22 トヨタ自動車株式会社 ガス濃度検出装置
EP2083263A3 (de) * 2008-01-24 2015-09-02 NGK Spark Plug Co., Ltd. NOx-Sensor und Herstellungsverfahren dafür
JP5350671B2 (ja) * 2008-04-28 2013-11-27 日本特殊陶業株式会社 燃料電池用水蒸気センサの異常検出装置、燃料電池用水蒸気センサ、及び燃料電池システム
JP4983726B2 (ja) * 2008-05-29 2012-07-25 トヨタ自動車株式会社 ガス濃度センサの暖機制御装置
JP4954185B2 (ja) * 2008-11-17 2012-06-13 日本特殊陶業株式会社 ガスセンサシステムと、ガスセンサの制御方法
JP5907345B2 (ja) * 2012-02-03 2016-04-26 株式会社デンソー ガスセンサ制御装置及び内燃機関の制御装置
US9164080B2 (en) 2012-06-11 2015-10-20 Ohio State Innovation Foundation System and method for sensing NO
JP6130184B2 (ja) 2013-03-27 2017-05-17 日本特殊陶業株式会社 センサ制御装置およびガス検知システム
JP6241360B2 (ja) * 2014-04-23 2017-12-06 株式会社デンソー 排出ガスセンサのヒータ制御装置
JP2016020892A (ja) * 2014-06-17 2016-02-04 株式会社デンソー 制御装置
WO2017047511A1 (ja) * 2015-09-17 2017-03-23 株式会社デンソー ガスセンサ
JP6369496B2 (ja) * 2015-09-17 2018-08-08 株式会社デンソー ガスセンサ
JP7088073B2 (ja) * 2019-02-19 2022-06-21 株式会社デンソー ガスセンサ制御装置
KR102539118B1 (ko) * 2021-08-19 2023-06-02 한국광기술원 광이온화 검출기
WO2023181582A1 (ja) * 2022-03-25 2023-09-28 ローム株式会社 センサシステム

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EP1202048A3 (de) * 2000-10-31 2004-06-02 Denso Corporation Gaskonzentration-Messvorrichtung mit Kompensation einer Fehlerkomponente des Ausgangssignals
EP1684067A3 (de) * 2000-10-31 2007-02-28 Denso Corporation Vorrichtung zur Messung von Gaskonzentrationen mit Ausgleichsfunktion für die Fehlerkomponente des Ausgangssignals
FR2825469A1 (fr) * 2001-05-31 2002-12-06 Denso Corp Systeme de controle d'alimentation electrique pour dispositif de chauffage utilise dans un capteur de gaz
EP1331478A1 (de) * 2002-01-24 2003-07-30 Volkswagen AG Verfahren zur Bestimmung der NOx-Konzentration in Abgasen
WO2007099647A1 (en) * 2006-02-28 2007-09-07 Toyota Jidosha Kabushiki Kaisha Temperature control apparatus for heater-equipped sensor
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Also Published As

Publication number Publication date
US6453724B1 (en) 2002-09-24
EP1764613B1 (de) 2014-12-31
EP1764613A3 (de) 2007-08-08
JP3983422B2 (ja) 2007-09-26
JP2000171439A (ja) 2000-06-23
EP1764613A2 (de) 2007-03-21
EP0995986A3 (de) 2001-04-18

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